A vehicle powertrain typically includes an engine operatively connected with a transmission that provides multiple speed ratios between the transmission input member and the transmission output member. Flex plates or flywheels are often provided as a drive connection between the engine output member (e.g., the crankshaft) and the transmission input member. Dampening mechanisms such as torsional springs are sometimes provided to isolate torsional oscillations of the engine, preventing their effect on the smooth feel of the transmission. A selectively engagable dampener bypass clutch is sometimes provided to directly connect the engine output member with the transmission input member, such as when the transmission is a hybrid transmission and the engine may be started by a motor/generator, to reduce the tendency for the torsional springs to oscillate during engine startups and shutdowns. An electric starter motor is sometimes used to start the engine and may be drivingly connected with the engine by moving a pinion gear into meshing engagement with a ring gear that is connected for common rotation with the flex plate.
Electromechanical transmissions having two motor/generators are often configured so that one of the motor/generators can provide driving torque for the engine output member during engine-off operation, but the other motor/generator requires reaction torque from the input member to also provide driving torque to the output member.